If you think about it, what you really mean is that it’s too precise. ie; the attacker has to have their arm in exactly position A, otherwise technique B wont work.

100% foolproof [insert weapon/body part etc here] defence is not a realistic goal, exactly because of all those ‘what ifs’ you’ve mentioned. Don’t be lulled into a false sense of security.

The ‘improvisation’ you’re talking about is the goal of self defence and martial arts in general. You need to respond without thinking to the situation as it evolves, and it will evolve and change far, far too fast for you if you’re trying to remember a technique from amongst a list of hundreds that you’ve memorised.

On the other hand; if you have learned and practice a set of principals and internalised them, then from these you can rely on your self to adapt spontaneously to the fight.
eg: If someone taught you that 2 + 3 is 5 and didn’t explain the function of the ‘+’ sign then you would only ever be able to answer 2+3, you’d be stumped when some one asked you to do 5 + 7!! however, if they taught you the principal that the ‘+’ means to add the values together, you’d be able to do any addition you wanted.

But, you can’t just be told that, you need to put in the practice adding things before it comes naturally.

to bring this back to martial arts; the first 2 + 3 you do is like this ‘knife disarm’ technique you’re learning; you can only do the disarm successfully within very fixed parameters.
After a while and you move on to disarm number 2 and 3 etc. you are still confined to the new parameters of the techniques.
After a number of these techniques you will start to see a common theme, the principals of knife defence. This is where you learn what the ‘+’ does, and how you can make it yours.

These principals that you should be trying to discover are not set in stone. some people may like to trap the knife close to there body, some may like to deflect it and counter attack. each has it’s benefits, but the only measure of correctness is if it works for that person.

It’s from these principals come the answers to the ‘what ifs.’

Remember, the overriding goal is to stop the person from harming you. however you get there is good, as long as you do get there.

Shit, this turned out to be quite long. It’s an important point though. The take home message is really run as fast away as you can if someone comes at you with a knife (Principle number 1!)

Good luck in the training!